The present invention relates to the art of earth boring and, more particularly, to an improved core drilling and recovery system.
It is common practice to take samples or cores of earth formations to obtain geological information. The cores are obtained by the use of a hollow rotary drill string or drill stem having a core bit at the lower end and a core barrel positioned within the hollow rotary drill string adjacent the core bit. When the drill string is withdrawn from the borehole, the core may be removed from the core barrel for analysis. It is also known to use a retractable core barrel to obtain the core sample without removing the drill string from the borehole. The retractable core barrel is locked in cooperative relation with the core bit until the core sample is taken. At that time, a retriever connected to a wire line is utilized to remove the core barrel by drawing it out of the drill string.
One of the major problems in obtaining an undisturbed core sample occurs after the drilling operation has been completed and the core sample is to be removed from the core barrel for measuring, inspection, sampling and laboratory testing. When using the conventional double-tube core barrel, the core sample must be slid or pushed from the inner tube of the core barrel and laid out in core boxes. When drilling soft or unconsolidated formations, extruding the core sample from the inner tube in substantially every instance either compacts the core sample or causes it to collapse. The transferring of the core sample to core boxes simply creates another potential source for damage or core sample loss. If the core is from a formation which tends to swell once the core is in the inner tube, for example in fire clay formations, great difficulty is experienced in removing the core sample from the inner tube. Mechanical or hydraulic core extruding devices have been employed. They apply a considerable axial load to the core to force it from the inner tube and, as a consequence, result in damage to the core sample. It will be appreciated that a need exists for a low-cost, simple and efficient system for obtaining undisturbed core samples. Such a system is especially needed for use in soft, friable formations, particularly coal measures.